The Oil Rig Turntable

sexta-feira, 13 de fevereiro de 2009

After coming across some DIY efforts at building turntables, I too decided to give it a try…For the turntable, I went for an all aluminium construction and a decidedly “mechanical” look.The frame consists of 2 aluminium sheets strongly bolted together, and the platter is a “sandwich” of aluminium-acrylate-aluminium epoxyed together for some vibration dampening.

As for the drive I went for what I believe to be an original path.

A bipolar stepper motor drives the platter through a toothed belt (to prevent slippage) at a 3:1 ratio.However, steppers are not known for a smooth motion which is absolutely critical here.

Here, it is driven by a special purpose Toshiba chip that generates two “pseudo” sinewaves 90 degrees apart (aka microstepping), to drive the stepper coils, thus strongly reducing any vibrations.

A Picaxe 28X1 controller running at 20Mhz provides the timing inputs to that chip with the required precision, both at 33 and 45 rpm.

The Tangential Tonearm

But the tonearm is the "piece de resistance"...

During the early 80's, tangentialtonearms were all the rage, mostly for top of the range models. At the time, I admired their sophistication but I was clueless at how they worked.Nowadays, thanks to the web, few secrets remain. As a personal challenge, I decided to make one myself, based on the workings of the Denon Linatrack.

In it, the cartridge was suspended from a very short arm that could rotatehorizontally from a carriage assembly that would move linearly along the record radius.

While following the record groove, the needle would pull the arm from its straightahead position, an optical switch would detect the motion and command a small motor to pull the carriage assembly along using a wire guide, thus restoring perpendicularity of arm and cartridge. So, even a small tracking error leads to an immediate correction.

My tonearm follows the same principle, although with a different mechanical layout. Here, the carriage is attached to a toothed rubber belt pulled by a stepper. A photodarlington is used to detect the cartridge movement and 2 micro-switches signal the limits of the carriage travel along a linear guide.

User input is provided by a 4-way joystick, and the following features are supported: Auto-stop at the end of play, Auto start for LP's and Singles, Auto-Repeat, Up/Down, Linear movement and finally Stop.

Due to the number of I/O lines needed, I opted again for a 28X1 controller. The extra lines also allowed me to control the stepper using halfstepping, providing a bit of a smoother action from the stepper.

All the mechanical bits are attached to an aluminium frame for rigidity and everything is partly enclosed with an acrylic cover. The tonearm itself is a somewhat complex part and for ease of prototyping, it was drawn using Google Sketchup and 3D printed by the folks at http://www.shapeways.com/

The End Result

So, how does it sound ?

All in all, it sounds reasonably good, although it ain't ready for commercial release and reviews. There are some noise sources that remain and are harder to suppress. That's where the consumer electronics giants spent most of their R&D budget - the small important details !

From the turntable itself, there's a few noises coming from the type of bearing I've used for the platter spindle, and only redesigning it will bring a complete cure. The stepper used for the platter should have more torque and that means that startup to 45rpm can be tricky sometimes.

And lastly, I forgot to provide for some kind of tension adjustment for the drive belt, which on ocasion brings some "light" warble when playing. However, a simple fix is already in the works.

The tonearm works incredibly well and reliably. Only a very muffled sound can be heard on quieter parts, when the arm assembly moves. Guess that driving the stepper using microstepping would take care of that. That's a more complex circuit maybe for the future.

Electrical noise and its interference on the signal lines became a bit of an issue towards the end of the build. Inexperience made me overlook it initially, and later on I had to reroute and shield (with aluminium foil) some of the lines. Also, some more grounding was needed.

It took me about 10 months from concept to finish. I had to learn a lot to make it work but it was hugely rewarding when it all came together, and on top of that it even sounded good !

Downloads

As I don't intend to start the next Sony (or Samsung !), I make available some of the information needed if you want to try your own version. The Eagle file schematic for the arm controller can be found here, and the necessary software (Picaxe Basic) in here.